The present invention is directed to generation of small images. It has particular, although not exclusive, application to microlithography of the type used in etching patterns during the production of integrated circuits.
The production of integrated circuits involves etching patterns into workpieces. In one common method, the pattern to be etched is initially produced in a large form by computer-generated-graphics equipment. The large-form pattern, or source image, is then reduced photographically to produce a mask, and light is projected through the resultant mask to create an object image on a workpiece. The workpiece typically is a wafer of silicon, sometimes with a metal-oxide layer, coated with a layer of photoresist, a substance whose solubility in a given solvent is affected by exposure to light. The solvent is applied to the workpiece so as selectively to remove the photoresist, and the workpiece is etched in the resultant pattern.
As integrated-circuit manufacturers have required increasingly small feature sizes, two diffraction-related problems in this method have become apparent. The first occurs in the production of the mask. To achieve the requisite size reduction, the light used in the reduction process must travel significant distances, and diffraction thus tends to smear features whose sizes approach the light's wavelength. For a given wavelength, therefore, diffraction tends to limit the extent to which feature sizes can be reduced. Since diffraction is a function of wavelength, of course, smaller feature sizes can be achieved by using shorter wavelengths. Indeed, so long as shorter wavelengths are available, there is no theoretical limit to how narrow the exposed region can be made. But it is more convenient and less expensive to employ photons that can be controlled by conventional optical equipment.
Still, it is possible to reduce the wavelength by employing shorter-wavelength photons, such as those associated with ultraviolet light or X-rays, or by employing beams of massive particles, such as electrons or ions, that have energies associated with shorter wavelengths. For example, the so-called direct-write system uses electrons to create the mask pattern. Rather than attempt somehow to reduce a large-form image "electrographically," however, the typical direct-write system generates a single electron beam having a beam energy high enough that it can be focused to a sufficiently small feature size, and the desired image pattern is "written" by deflecting the beam in the desired pattern over the surface of a mask medium. The medium is coated with electroresist, which is to electrons what photoresist is to photons, and the medium is then etched in much the same way as the ultimate workpiece is.
Although this method eliminates the diffraction problem, it does so at a relatively high cost, since it does not use easy-to-control optical-wavelength light. Moreover, it still leaves a second diffraction-related problem, that of exposing the workpiece. If the mask could be brought into contact with the workpiece or extremely close to it, diffraction would not be a problem in workpiece exposure. However, practical problems prohibit bringing the mask close enough to the workpiece to eliminate the diffraction problem. The light that exposes the photoresist must therefore travel a significant distance between the mask and the workpiece, so diffraction keeps the feature sizes on the workpiece from being as small as those of corresponding mask feature that approach the wavelength of the exposing light. Of course, the direct-write method can be used on the workpiece itself, but it suffers from the same expense disadvantage in circuit production as it does in mask production. Moreover, it is much slower than illumination through a mask, and the speed disadvantage is a significant drawback in volume circuit production.
In short, diffraction constitutes a significant problem in microlithography. It also presents a similar problem in other microfabrication techniques and in image-reduction processes generally.